The present application relates generally to the field of aircraft radar systems. More specifically, the application relates to aircraft radar systems using pulse compression.
Conventional pulse Doppler radar systems emit a generally uniform, rectangular pulse of energy. The pulse reflects off a target and is received by the radar. By measuring the time between the transmitted signal and the received reflected signal the target range can be obtained. For airborne weather radars, the return signal strength is used to infer the rainfall rate (displayed as green, yellow, and red colors on the flight deck radar display) after being compensated for target range.
A radar pulse and the reflected return signal are not infinitely narrow, even from a point target. In a best case where the radar pulse reflects off a highly reflective point target (effectively a radar mirror) the reflected signal is the same duration as the transmitted pulse, but after going through the radar receiver electronics the received signal includes additional thermal noise and noise associated with the amplifier.
A radar receiver may include a signal accumulator called a “matched filter” to reduce the noise and thus increase the received signal to noise ratio. For purposes of this disclosure, a matched filter is a filter that has a frequency response equal to the complex conjugate of the frequency spectrum of the transmitted pulse. Other less complex and less exact filters are also used that only approximate the matched filter response. For example, the matched filter for a rectangular transmit pulse can be approximated by a running average accumulator with a running average duration equivalent to the length of the transmit pulse. This increases the available target signal to noise ratio compared to a un-filtered received signal. In this approximation for a matched receiver filter, a rectangular return pulse from a point target may be accumulated and then sampled at the end of the accumulation period. The highest sample value may occur when the signal is integrated over the duration of the transmit pulse. The bandwidth of this example rectangular pulse is roughly equal to 1/pulse length. This simple system has a figure of merit of the bandwidth times the pulse length approximately equal to one. A general discussion of pulse lengths and bandwidths for simple pulse systems can be found in chapter 10 of the second edition of Stimson's “Introduction to Airborne Radar”, SciTech, 1998.
If the transmit pulse length (and matched filter accumulator duration) is increased, more return signal power can be accumulated in the matched filter. The matched filer for these longer pulses decreases in bandwidth with a B*T ratio still approximately equal to one. The decrease in matched filter bandwidth allows less thermal noise to be processed along with the desired signal. Therefore, increases in transmitter pulse length increase the signal to noise ratio of the received radar signal because both the amount of energy has increased in the transmitter pulse and the amount of noise the desired signal must compete with decreases as the bandwidth is decreased. By increasing the pulse width, the radar system may operate with less transmit power, detect less reflective objects, detect objects at longer ranges, or any combination thereof.
However, if the transmit pulse length is increased, the overall resolution in range decreases. After a very long transmit pulse is accumulated in the matched filter, the aircraft radar system may have difficulty differentiating between reflections that occur at the beginning of the pulse and reflections that occur at the end of the pulse. All the signals are merged into an average in the matched filter.
Pulse compression is a signal processing technique than can be used in radar systems to augment the radar range resolution as well as the signal to noise ratio by modulating the transmitted pulse and correlating the received radar return signal with the transmitted pulse. While compression of radar signals allows higher range resolution to be obtained with longer transmitter pulses, it conventionally has a problem of producing minimum detection ranges much in excess of what a short pulse radar can produce.
Conventionally, increasing the resolution of long pulse radar (i.e., longer than 10 microseconds) is done by modulating the transmitter pulse with some signature and then demodulating the received radar pulse with a matched filter for that same modulation signature. The modulation signature may take many forms including frequency compression, binary coded phase compression, and polyphase coded systems. The modulation process increases the spectral bandwidth occupied by the transmitter pulse.
While the modulation allows higher range resolution than the original long pulse, the entire modulation sequence is used in the demodulation of the received radar signal. If the entire modulation sequence must be sampled, targets must produce returns from the entire sequence. Consider a 100 microsecond long pulse with a 100 microsecond long modulation sequence. If receiving the pulse commences at just after the 100 microsecond long transmitter pulse, radar returns representing the entire modulation sequence can only be sampled at 100 microsecond/12.36 microsecond per nautical mile away, or about 8 nautical miles. Targets at a lesser range will not have produce radar returns containing the entire modulation pattern since those portions of modulated returns could not be received while the radar is still transmitting.
Therefore, what is needed is a radar system capable of using pulse compression to allow long pulse lengths while achieving both high range resolution and producing return power estimates from ranges shorter than either the pulse length or the matched filtered return pulse length. What is also need is a radar system and method that can accurately differentiate between reflections that occur at the beginning of the pulse and reflections that occur at the end of the pulse while increasing the signal to noise ratio, operating with less transmit power, detecting less reflective objects, detecting objects at longer ranges, or any combination thereof.